An advanced digital cordless telephone (DCT) system uses digital, time divided, transmissions referred to as TDD (time division duplexing). The transmissions occur between one or more portable hand held units (typically pedestrians or persons moving around in an office building) and a base station which serves a defined geographic area known as a cell. A typical operational frequency for a DCT is in the vicinity of 2000 MHz. The base station typically transmits a burst of data every 5 milliseconds and the portable handheld unit transmits back a burst of data about 2.5 milliseconds after the base station transmits. Both units transmit at the same frequency, and transmit bursts are typically about 625 microseconds (including guard time) as shown in FIG. 1. Based on these timing constraints the DCT system can provide four channels so that four communications links can be established between four portable units and the base station. Each of the portable units is assigned one of the 625 .mu.sec. slots in each of the 2.5 msec. receive and transmit windows.
In an urban or indoor environment there is significant multipath effect with transmissions at this frequency range, which manifests itself as Rayleigh fading of the received signal level. One technique used to minimize this problem is known as diversity. The underlying idea behind diversity, as an antidote to Rayleigh fading, is that if one is able to obtain reception via two (or more) independent paths then it is unlikely that both (or all) of these paths will fade simultaneously. These independent paths may be obtained by diversity in time, frequency, or space. Since the DCT operates in TDD, time diversity is not a good solution to correct the multipath problem since implementing time diversity in the DCT would require at least twice as much time to be allotted to each channel. Similarly, frequency diversity is problematic for a TDD system in that both base stations and portable handheld units are designed to operate at only one frequency.
Therefore, to combat this fading, the base station might include two or more antennas. Each antenna receives a different standing wave pattern. The base station attempts to select the antenna with the stronger signal path. This technique is referred to as space diversity. Some typical signal levels versus location plots are shown in FIG. 2. As long as the portable unit moves a small percentage of the fading distance in a frame time (equivalent to a burst), the ideal signal path will be the same for both base station and the portable unit within one frame or burst time.
It is possible to build two entire receive chains (i.e., hardware and software required to detect, demodulate and decode the received signal) in the base station radio, and after each burst is received use data from the burst with the higher signal strength or the lowest error rate. This technique, often called selection diversity, works very well. Although, selection diversity may be very effective in combatting the effects of fading, it has several drawbacks.
First, providing a separate receive chain for each of the antennas drives up the cost and complexity of the system. If it is desirable to provide a low cost processor to provide the switching control, a separate receive chain for each antenna may be prohibitive. Furthermore, the added complexity in evaluating the signal as received and demodulated by each receive chain adds to the cost of the processor and requires more complicated programming.
Alternatively, multiple antennas may be used with a single receive chain. However, these diversity systems although less expensive and less complex to implement, create other design problems. For instance, almost all radio receivers implement some form of gain control to prevent saturation and degradation due to intermodulation when the received signal level is too high or to increase the signal-to-noise ratio of the received signal when the received signal level is too low. Since the signal level received by each antenna may vary, the gain required for each antenna may also vary. In the past, determining the proper gain adjustment for a selected antenna could only begin after the antenna was actually switched to an ON state. Therefore, it has been virtually impossible to effectively combine antenna switching and gain control in a single receive chain implementation.
Furthermore, previous antenna diversity systems having a single receive chain have used an averaging technique to estimate the signal levels at the OFF antennas. Such averaging techniques require that estimates for a number of previous frames be stored in memory. For instance if N is the number of frames to average over, the estimated average signal level y{n} would be: EQU y{n}=y{n-1}+1/N*x{n}+1/N*x{n-N}
where x{n} is the measured signal level of the received signal. It is evident from the above equation that N measured signal levels must be stored to carry out the averaging. As N increases, so does the memory requirements and the implementation costs.
Another disadvantage of switch diversity using a single receive chain is that when a signal on one antenna is weak but usable, the signal on the other antenna may be weaker. In that event it would be undesirable to switch to the other antenna merely to determine that the received signal from that antenna is not only just as weak but unusable. Some previous diversity systems have provided some control to enable or disable switching if the received signal is determined to be acceptable. However, such systems have merely considered the signal level of the received signal in determining whether the received signal using the currently selected antenna is acceptable. Typically, other characteristics of the received signal should be considered to avoid unnecessary and ineffective switching.
Therefore, there is a need for a method providing antenna diversity and gain control in a single chain implementation which is both low in cost and simple to implement without substantially reducing the performance of the communications.